Object: Earth-sized exoplanet
Composition: Rock and iron
Temperature: Scorching
Earth now has a twin – but it's evil. There's a planet the same size as Earth in a distant solar system, and it shares our planet's mass and composition. However, the rocky exoplanet is so close to its star that it orbits it once every 8 hours, making it hellishly hot, with almost no prospect of hosting life.
The planet, Kepler-78b, is not the first world to come within 20 per cent of Earth-like size or mass – those distinctions go to Kepler-10b and MOA-2007-BLG-192-L b, respectively.
It's not even the first to claim a rocky make-up, a title that went to COROT-7b in 2009. Kepler-10b is also thought to be rocky. But it is the first to have all three characteristics at once, and raises astronomers' hopes that life-friendly Earth twins are out there.
"This is an existence proof," says Andrew Howard of the University of Hawaii in Honolulu. "When you have one, you know that Nature can make Earth-sized rocky planets outside of the solar system."
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